1. Field of the Invention
This invention relates to roofing compositions having enhanced solar reflectance. More particularly, the invention relates to roofing granules having enhanced solar reflectance and to roofing shingles incorporating said granules.
2. Reported Developments
Roofing granules, both natural and artificially color-coated granules, are extensively used in roll roofing and asphalt shingle compositions. The roofing granules are typically embedded in the asphalt coating on the surface of an asphalt-impregnated felt base material, the granules thus forming a coating that provides a weather-resistant exterior roofing surface. In addition to weather-resistance, the color-coated granules also provide an esthetic effect observable with respect to the coating compositions; the appearance of the granules is of major marketing interest (see, for example, U.S. Pat. No. 6,607,781). For this reason, a pigmented color coating is ordinarily applied to the base mineral granules. In addition to pigments, the granule coatings may contain additives such as algaecides to help extend the color and appearance of the roofing material.
In recent years interest in mineral-surfaced roofing of increased solar reflectance has gained momentum as a way to reduce summer cooling costs and to mitigate smog-producing urban “heat island” effects. The EPA Energy Star Initiative requires steep-slope residential roofing to have a minimum Total Solar Reflectance (TSR) of 0.25 (25%). Conformance to this requirement is already possible for many light-colored shingles and blends, but most existing popular dark-colored roofing products have TSR well below 0.25. There is, therefore a great need for dark roofing granules of enhanced solar reflectance that will allow production of residential steep-slope mineral—surfaced roofing products that can meet the EPA Energy Star requirements. Illustrative examples of the prior art follow.
U.S. Publication No. 2005/0074580 relates to a non-white construction surface having reflective coatings for enhancing solar reflectance for use on roofs, such as on asphalt shingles. The method of manufacture is a “bilayer” process, i.e. it involves two distinct coatings: a first coating on the outer surface of a substrate to achieve a direct solar reflectance of at least 25%; and a second coating applied over the first coating to achieve a final TSR of at least 20%. Both coatings use solar reflective materials therein.
U.S. Publication No. 2005/0072114 relates to solar-reflective roofing granules of deep-tone colors formed by coating base mineral particles with a coating composition including an IR-reflective pigment. Color is provided by colored infrared pigments, light-interference platelet pigments, or metal oxides. Both single-coat and bilayer coatings are described.
U.S. Publication No. 2005/0072110 relates to roofing shingles with increased solar heat reflective and a process of making such shingles. The infrared reflective material can be applied to the shingles as a powder in a carrier fluid or a film. Infrared reflective granules can be applied along with the IR-reflective powder.
Simple replacement of existing “hot” pigments, such as Carbon Black and Black Iron Oxide, with new IR-reflective mixed metal oxide pigments during roofing granules manufacture is only partially effective in raising the solar reflective properties of dark roofing granules due to the significant loss of reflectance caused by multiple reflections as a result of the rough granular surfacing of standard asphalt shingles and roll products. Use of the bi-layer or two-coating process in which a highly IR-reflective bright white undercoat is covered by a pigmented overcoat, is effective in enhancing the solar reflectance of roofing granule products. In this two-coat system, IR radiation not reflected or absorbed by the pigmented overcoat is reflected by the undercoat to enhance total IR reflectance. However, this process is very expensive and consumes manufacturing capacity since these roofing granules must be processed twice in order to apply the two coatings. In addition, the resulting product has a very spotty appearance with the bright white undercoat showing through portions of the dark-colored overcoat. The color of such granules would be expected to lighten over time as the outer coat weathers away and more undercoat is exposed. What is needed is an improved process to efficiently and cost-effectively manufacture dark-colored roofing granule products of uniform color, appearance and enhanced solar reflectance.
It is an object of the present invention to produce roofing granules that generally have a minimum of Total Solar Reflectance (TSR) of 0.25 (25%) to meet the EPA Energy Star Initiative. However, roofing granules darker than a Hunter L-value of 25.0 will have a minimum TSR of 0.20 (20%).
It is another object of the present invention to meet the Total Reflectance requirement in non-white colored roofing granule products to satisfy visual appearance requirements.
It is a further object of the present invention to make possible the use of a one coat coloring process system on granule products to minimize the cost of manufacturing.
These and other objects will be addressed as the description of the invention proceeds.